Please referring to FIG. 1, a conventional power adapter (or charger) generally includes a transformer 101 which has circuits divided into a primary side and a secondary side. The primary side has a power switch 103 and a pulse-width modulation (PWM) unit 100. The PWM unit 100 generates a driving pulse to set the power switch 103 ON or OFF. Through a coupling element 102 a feedback signal is sent from the secondary side to the primary side to control energy sent through the transformer 101 to the secondary side. The secondary side delivers output through a cable to an external electric device. However to deliver output at a constant voltage the cable will incur about +/−10% of voltage drop. Hence on the conventional circuit the secondary side usually has an impedance compensation circuit 104 to alter the impedance of the output end to attain the constant voltage. Such an approach requires more elements and results in a higher cost. The circuit board also is bigger. There are still rooms for improvement.